1. Field of the Invention
The present invention relates in general to a switching circuit, and more particularly, to a switching circuit of a ballast.
2. Description of Related Art
Fluorescent lamps are one of the most popular light sources in our daily lives. Improving the efficiency of fluorescent lamps saves energy significantly. Therefore, in recent developments, the improvement of the efficiency and power savings for the ballast of the fluorescent lamps are the major issues. FIG. 1 shows a conventional electronic ballast circuit in series connection with a resonant circuit. A half-bridge inverter consists of a first switch 10 and a second switch 20. The two switches 10 and 20 are complementarily switched on and off with 50% duty cycle at a desired switching frequency. The resonant circuit is composed of an inductor 70, a capacitor 75 to operate a fluorescent lamp 50. A capacitor 55 connected in parallel with the fluorescent lamp 50 operates as a start-up circuit. Once the fluorescent lamp 50 is turned on, the switching frequency is controlled to produce a required lamp voltage. The drawback of this ballast circuit is higher switching losses on the switches 10 and 20. The parasitic devices of the fluorescent lamp 50, such as the equivalent capacitance, etc., vary in response to temperature variations and the age of the fluorescent lamp 50. Besides, the inductance of the inductor 70 and the capacitance of the capacitor 75 also vary during mass production of the ballast circuit.